The present invention is related to the field of electronic circuit components and component packaging, and more particularly to electronic circuits using high-current, low-resistance current sensing resistors.
There is widespread use of current sensing resistors in various types of electronic circuits. These high-power, low-resistance resistors are commonly placed in series with a source of high current, such as a power supply output. The current causes a voltage proportional to the current to appear across the current sensing resistor. This current-indicating voltage is commonly referred to as a current sense signal. The current sense signal can be used by control circuitry whose operation varies in response to output current level. For example, an over-current protection circuit within a power supply can monitor output current via the current sense signal. The protection circuit may act to inhibit the power supply output as the output current level rises, in order to counteract the rise in output current level and thus keep it below a safe limit.
Typical existing current sensing resistors have resistance values on the order of 10-100 milliohms. These devices dissipate considerable power in the form of heat when operated at typical operating currents. For example, a 10 milliohm resistor operated at 10 amperes dissipates approximately 1 watt of power in the form of heat.
It is common for many or all of the components that constitute the circuit using the current sense signal to be fabricated on a single integrated circuit. An example of such a circuit is a control circuit for controlling the switching of a power transistor connected in series with the current sensing resistor. The control circuit uses the current sense signal from the current sensing resistor for any of a variety of purposes, such as over-current protection which prevents excess current from flowing through the switching transistor. The current sensing resistor is commonly a discrete component apart from the integrated control circuit. This configuration avoids the-problems of excessive power dissipation on the integrated circuit and excessively high current densities on the integrated circuit that may exceed the current-handling capacity of conventional metallization and bond wires.
However, the use of discrete current sensing resistors separated from their associated integrated control circuits has associated drawbacks. One drawback is cost. Generally speaking, the cost of an electronic circuit board increases as the number of different components on the board increases. Accordingly, it is generally desirable to integrate as many circuit elements as possible onto integrated circuits, so that the overall component count on a board can be reduced to minimize costs. Another drawback of using discrete sensing resistors is that circuit operation may be hampered by excessive inductance in the circuit board traces that interconnect the sensing resistor with the integrated circuit, and also by impedance mismatches between the integrated circuit and the resistor that occur over an operating temperature range. The problems of excessive inductance and impedance mismatches can be substantially eliminated when components exist on the same integrated circuit.
It would be desirable to achieve the performance benefits of integrating the current sensing resistor and its associated control circuit components on an integrated circuit while avoiding the problems of excessive power dissipation and excessive current densities on the integrated circuit.